The increasing input/output (I/O) interconnection density requirements for thin film packages suggest that the current techniques for producing terminal metal pads, such as evaporation through masks and screening, will not be able to meet the tighter ground rules of the advancing technology. This concern is expected to be especially true for land grid array (LGA) type interconnect applications where the typical ground rules for high end packages are in the range of 0.2 to 1.2 millimeters for pad diameters, with a minimal pitch in the range of 0.25 to 1.3 millimeters.
Terminal metal pads require certain characteristics to function as contact points for thin film packages. Typically, a wetting material such as gold (Au) is needed as an outermost metal film to which an electrical connection is provided. Also required is a metal such as nickel (Ni) which serves as a barrier and provides strength. Additionally required as a processing necessity is copper (Cu). Copper is a very ductile material, and acts as a cushion to absorb the residual stresses from a film such as nickel. Unfortunately, the required presence of a film such as copper raises corrosion concerns: copper is susceptible to corrosion when it combines with the moisture present in the air. Because copper is typically exposed at least in the sidewalls of a terminal metal pad, the presence of copper makes the pads susceptible to corrosion.
The currently available processes for producing terminal metal pads may include the following limitations: (1) they are applicable only for pads in a low-density pattern and cannot be scaled to high-density patterns; and (2) they produce multilayer patterns only in the vertical direction, thus leaving exposed the sidewalls of the lower, usually corrosion-suspect metals.
Currently available processes for producing terminal metal pads such as: (a) evaporation through a contact mask; and (b) screening through a mask before ceramic sintering, followed by electroless plating, cannot produce pads in the high-density patterns necessary in modern technology.
To meet the tight packaging requirements required today and in the future, the process of photolithographically patterning a film is the only conventionally known process for producing the high-density pattern of closely spaced pads needed. Using this process, a photolithographic pattern is formed on top of a metal film structure, which may include multiple metal films formed on top of one another. The photolithograph pattern includes a masked region and an exposed region. Next, an etching procedure or series of procedures may be used to remove the film or films in the exposed region and produce a plurality of discrete terminal pads. In this manner, however, the underlying metal films are exposed laterally on the sidewalls of the terminal pads. Therefore, the terminal metal pad may have a structure whereby the films are layered only in the vertical direction and each film is exposed in the lateral direction along the sidewalls. Using this process, when copper is used as an underlying cushioning film, it is included as one of the films exposed in the sidewall, producing a corrosion susceptibility concern in the pads.
The contemporary processes available which can provide sidewall coverage use the electroless plating approach. Traditionally, the process of electroless plating is limited in its versatility; only a limited number of metals can be conveniently deposited using this procedure. Electroless plating is also a relatively expensive, time-consuming process. In addition, the process control for electroless plating is rather complicated due to the relatively narrow process window. Because of poor uniformity characteristics and poor process control, electroless plating is an approach not suited to high-density patterns produce by photolithography.
In the current technology for producing metal terminal pads, the elimination of copper from the terminal pads does not loom as a viable alternative. Without copper, the pads would be susceptible to failure due to cracking of the substrate and/or the more brittle metals that are used to fabricate the pad. Contemporary processes for producing pads include the limitations discussed above. A possible alternative process, for producing corrosion-free pads capable of meeting modern packaging density requirements, would be to deposit a passivation layer on top of the metal pads after the metal pads have been formed. Once deposited, the passivation layer must be patterned and openings must be created to expose only the top of the terminal pads. The shortcomings of this process include the addition and patterning of a separate passivation layer. Such passivation layers require additional processing materials and the additional time and expense associated with forming and patterning the passivation layer. Furthermore, a passivation layer may not be an option for some packaging technologies including those applications for which the terminal pads need to be raised and cannot be "buried."
What is needed is a process which overcomes the shortcomings of contemporary processing technology options, and provides a process for producing tightly packed terminal metal pads capable of meeting the increasing input/output interconnect density requirements. The individual pads produced by this process must be corrosion resistant and must include the strength, wettability, ductility, and cushioning characteristics required to enable the terminal metal pad to provide a reliable connection to an outside component. The process used to form the contact pads will most desirably be inexpensive, fast, reliable, and versatile.